Aldo Sottolichio

A synthesis on seasonal dynamics of highly-concentrated structures in the Gironde estuary

Two turbidity peaks can be identified in the Gironde estuary waters. The first one, in a steady position, is possibly related to a mud-trapping zone in the main channel. The second one is the actual turbidity maximum, capable of shifting seasonally along the estuary. Its length and position can be readily predicted as functions of the river flow. Fluid mud presents a similar seasonal pattern, but its occurrence is sensitive to the channel cross-section, especially to its width, which modifies local hydrodynamics and water-bottom exchanges. These observations allow accurate prediction of mud deposition zones according to the season.

Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France)

Dissolved oxygen (DO) is a fundamental parameter of coastal water quality, as it is necessary to aquatic biota, and it provides an indication of organic matter decomposition in waters and their degree of eutrophication. We present here a 7 year time series of DO concentration and ancillary parameters (river discharge, water level, turbidity, temperature, and salinity) from the MAGEST high-frequency monitoring network, at four automated stations in the central and fluvial regions of the macrotidal Gironde Estuary, one of the largest European estuaries. The spatio-temporal variability of DO at different time scales was first related to the migration and position of the maximum turbidity zone in this extremely turbid estuary. Since 2005, the Gironde Estuary has recorded several borderline hypoxic situations (DO close to 2 mg L(-1)) and a 7 day-long hypoxic event (DO < 2 mg L(-1)) in July 2006. Summer hypoxia occurred exclusively in the fluvial, low salinity, and high turbidity sections of the estuary and was significantly more pronounced in front of the large urban area of Bordeaux (715 000 inhabitants). Detailed analysis of the data at the seasonal, neap-spring and semi-diurnal tidal time scales, reveals that hypoxia in this area occurred: (i) in the maximum turbidity zone; (ii) during the spring to neap tide transition; (iii) at highest water temperature; and (iv) at lowest river discharge; there was also evidence of an additional negative impact of sewage treatment plants of Bordeaux city. Enhancement of respiration by turbidity, temperature and inputs of domestic biodegradable organic matter and ammonia, versus renewal of waters and dispersion of reduced pollutants with the river discharge, appeared as the dominant antagonist processes that controlled the occurrence of summer hypoxia. In the context of long-term environmental changes (increase in temperature and population, decrease in summer river discharge), the occurrence of severe hypoxia could not be excluded in the next decades in the upstream reach of the Gironde Estuary.

Modelling the effects of Zostera noltei meadows on sediment dynamics: application to the Arcachon lagoon

A three-dimensional model has been modified to describe the complex interactions between hydrodynamics, sediment dynamics and biological parameters in the presence of Zostera noltei. The model treats seagrass leafs as flexible blades that bend under hydrodynamic forcing and alter the local momentum and turbulence fluxes and, therefore, the benthic shear conditions; these changes cause related changes to the mass balance at the boundary of the bed, in turn affecting the suspended matter in the column and ultimately primary productivity and the growth of the dwarf-grass. Modelling parameters related to the impact of Z. noltei to the local flow and to erosion and deposition rates were calibrated using flume experimental measurements; results from the calibration of the model are presented and discussed. The coupled model is applied in the Arcachon Bay, an area with high environmental significance and large abundance of dwarf-grass meadows. In the present paper, results from preliminary applications of the model are presented and discussed; the effectiveness of the coupled model is assessed comparing modelling results with available field measurements of suspended sediment concentrations and seagrass growth parameters. The model generally reproduces sediment dynamics and dwarf-grass seasonal growth in the domain efficiently. Investigations regarding the effects of the vegetation to the near-bed hydrodynamics and to the sediment suspension in the domain show that dwarf-grass meadows play an important part to velocity attenuation and to sediment stabilisation, with flow and suspended sediment concentrations damping, compared to an unvegetated state, to reach 35–50 and 65xa0%, respectively, at peak seagrass growth.

Utilisation of numerical and statistical techniques to describe sedimentary circulation patterns in the mouth of the Gironde estuary

Abstract Sediment distribution is well defined in the mouth of the Gironde estuary but the sediment dynamics is still poorly understood. The aim of this work is to analyse residual circulation of non-cohesive sediment due to the action of the main hydrodynamic factor: tidal currents. A depth-averaged numerical model emphasises the residual circulation of the Gironde estuary, which is characterised by residual tidal gyres. The comparison between sediment transport calculated from the numerical model and from a statistical analysis of the sedimentary distribution leads to the definition of a simple scheme of non-cohesive sediment circulation in the Gironde estuary’s mouth.

Modeling mechanisms for the stability of the turbidity maximum in the Gironde estuary, France

Two numerical models are applied to the Gironde estuary in order to study basic mechanisms of turbidity maximum formation. A 2D depth-averaged model reproduces the turbidity maximum location for different river flow conditions, wherein tidal wave propagation is the only hydrodynamic forcing. Yet, excessive sediment escape to the ocean and associated loss of mass of the tidal turbidity maximum are observed. A 3D model is applied in order to incorporate vertical gradients and density stratification. The turbidity maximum is better reproduced, especially with respect to lateral gradients. The introduction of salinity gradients leads to less seaward dispersion, without any modification of the structure and location of the turbidity maximum. Results suggest that the turbidity maximum in the Gironde is exclusively tidally-induced, while density induced residual circulation imparts stability to the suspended sediment mass in the estuary. This conclusion is preliminary, as sedimentary processes are simplified in the model, and validation of sedimentary patterns is qualitative. In addition, the observation regarding the salinity effect needs to be verified by longer simulations with the 3D model, in order to provide an assessment of seaward fluxes over long periods.

Evaluation of the recent morphological evolution of the Gironde estuary through the use of some preliminary synthetic indicators

ABSTRACT Sottolichio, A., Hanquiez, V., Périnotto, H., Sabouraud, L. and Weber, O., 2013. Evaluation of the recent morphological evolution of the Gironde estuary through the use of some preliminary synthetic indicators. In: Conley, D.C., Masselink, G., Russell, P.E. and OHare, T.J. (eds.) Drastic evolution of the physical coastal environment are expected in the future under the effect of climate change. They make necessary to better understand the past morphology evolution, especially considering areas that are the habitat for some specific ecosystems. Moreover, because the context of the water framework directive (WFD), EU members need to generate indicators to evaluate the status of estuarine water masses. To satisfy these two obectives, the recent evolution of the Gironde estuary has been studied by the analysis of bathymetry with GIS. The Gironde is the largest estuary of western Europe, and one of the most turbid. In this study, the analysis extents from 1962 to 1994. Results show that the zone of maximum volume of deposited sediment has migrated continuously towards the upstream portion of the estuary, which is coherent with the decrease of summer river flow and the upstream shift of the turbidity maximum toward the riverine sections. In addition, zones with relative stable and unstable morphology were identified, showing rythmic distribution similarly to those previously recognized through the evolution over 160 years (1825-1984). This seems to be independent from the fluvial regime, but rather related to the interaction between tidal cooscillations and estuarine morphology, which is not elucidated yet. Finally, some hydro-morpho-sediemntary (HMS) indicators useful to the WFD have been described : distribution of depths, changes on cross section areas, changes of intertidal areas. They are discussed to discriminate the “natural” and “anthropogenic” contribution to morphological changes observed.

Observations of waves' impact on currents in a mixed-energy tidal inlet: Arcachon on the southern French Atlantic coast

ABSTRACT Sénéchal, N., Sottolichio A., Bertrand, F., Goeldner-Giannella, L., Garlan, T., 2013. Analysis of currents and storm surge in a mixed-energy tidal inlet during storm conditions. Coastal morphodynamic processes around tidal inlets in mixed-energy environments are particularly complex due to severe tide and wave conditions. Collecting data in this area is generally very challenging. Here we present the first hydrodynamic data collected in the outer inlet of the Arcachon lagoon, a mixed-energy inlet situated on the southern part of the French Atlantic coast. Data consist in vertical current profiles and sea surface elevations collected on the offshore edge of the ebb delta under various tidal and wave conditions. In particular data were collected during the severe Joachim storm associated to wave heights up to 8 m. Preliminary results indicate that current profiles, intensities and direction are very sensitive to wave conditions. Under energetic conditions, the vertical profile of currents become uniform and the longshore drift is enhanced (up to 1.5 m/s). Nevertheless data show that under severe wave conditions (Hm0 > 6.0m) and despite high incidence of waves, the longshore drift is weaker (l< 1 m/s) than under ‘usual storm conditions associated to Hm0 = 4 – 5m. More surprisingly, directions of the longshore drift are not always consistent with wave incidence under those severe storm conditions and the cross-shore components are very weak while under less energetic conditions we observe intense offshore currents (up to 0.8 m/s).

Wind‐driven modifications to the residual circulation in an ebb‐tidal delta: Arcachon Lagoon, Southwestern France

A combination of observations and analytical solutions was used to determine the modifications caused by wind forcing on the residual or nontidal circulation in an ebb-tidal delta. Observations were obtained in the lower Arcachon Lagoon, southwestern France. The basic nontidal circulation was established with acoustic Doppler current profilers (ADCPs) that were (i) moored in the deltas two deepest channels, and (ii) towed along a cross-lagoon transect. The bathymetry of the lower lagoon, or ebb-tidal delta, featured two channels: North Pass (>9 m) and South Pass (>20 m). The basic nontidal circulation consisted of mostly inflow with weak surface outflow in the South Pass, and laterally sheared bidirectional flow, dominated by outflow, in the North Pass. Analytical solutions and comparison of observed dynamical terms suggested that, in addition to the conventionally accepted influence of tidal nonlinearities, density gradients contributed to the basic nontidal circulation in the lagoon. Observations also indicated that wind forcing altered the basic circulation by driving simultaneous upwind flows in both passes. This response was supported by an analytical solution to wind-driven flows over the bathymetry of the transect sampled. The response to seaward winds was to enhance inflow in South Pass and reduce outflow in North Pass. Landward winds caused diminished inflow in South Pass and increased outflow in North Pass.

To What Extent Multidecadal Changes in Morphology and Fluvial Discharge Impact Tide in a Convergent (Turbid) Tidal River

Understanding nonstationary tides in tidal rivers is a major contemporary challenge. In particular, the response of river tides to natural developments in the estuary remains poorly investigated. This study analyzes the evolution of tidal characteristics over the last six decades in the Garonne Tidal River (GTR, SW France), in order to explore the effect of natural and human-induced morphological and hydrological changes on river tides. The tidal Garonne is an excellent example, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. Tidal range (TR), distortion (AM4/ AM2), and asymmetry direction (2/M2-/M4) were calculated at four locations from the water level time series of 1953, 1971, 1982, 1994, 2005, and 2014. The annual time series of M2 and M4 amplitudes and phases were obtained through complex demodulation. Results reveal that both TR and 2/M2-/M4 have increased since the 1950s. River flow modulates TR and AM4/AM2 significantly. A long-term decrease in summer discharges from 200 6 50 to 100 6 50 m s would increase TR by 111.5% in the upper GTR. Natural morphological changes amplified TR and 2/M2-/M4 by up to 112–15% between 1953 and 2014. TR and 2/M2-/M4 doubled in the regions affected by gravel extraction between 1953 and 1971. Further, the persistence of mobile mud in the GTR increased TR seasonally but also interannually (by up to 116% in winter and spring of dry years). The potential impact of these changes on suspended sediments is discussed, revealing complex feedback between the evolution of hydrology, morphology, tides, and sediment trapping.

Leaf litter degradation in highly turbid transitional waters: preliminary results from litter-bag experiments in the Gironde Estuary

The rates of decomposition of oak (Quercus robur) leaves have been examined using litter bags in a very high turbidity macrotidal estuary, the Gironde Estuary (S.W. France). The first experiments show a marked decrease in the decomposition rate of oak leaves at the water-sediment interface (mud-contact: anoxic conditions, reduced physical fragmentation) in comparison to the water column. The results point out the impact of hydrodynamic conditions on leaf litter degradation in such fluvio-estuarine systems. Regarding the aquatic-terrestrial linkage, our observations suggest direct changes in leaf decomposition kinetics and then, a potential delay on the recycling and transport processes of coarse particulate organic matter, especially in a context of modification of the natural water flow, due to global and land use changes.